Conventionally, a so-called flexible flat cable (Flexible Flat Cable; hereinafter referred to as FFC) is often used as a connecting cable internally disposed within various electrical products, especially printers and scanners. Due to its superior flexibility, the FFC can be used in moving parts and furthermore, when compared to the flexible print circuit (Flexible Print Circuit; FPC), has a lower manufacturing cost which leads to lower cost per unit, making it applicable in a wide range of fields.
It is to be noted that conventionally, the FFC did not require any type of characteristic impedance or electrical characteristics. Due to this the FFC, as shown in FIG. 1, has a core conductor 101 affixed from both sides by the base film 103, made of polyethylene terephthalate and the like attached to the fixed adhesive layer 102, and when laminated is able to fulfill the necessary specification requirements solely through the adhesion of the base film 103 on both sides.
To the contrary, in recent years the development of electrical products that realized increased high definition quality of graphics such as notebook style personal computers and digital scanners has been accompanied by a demand for an increase in the speed of signal transmissions. Furthermore, even in the case of other electrical products, as progressing towards digitalization, those products raise imperative technological problems in increased speed of signal transmissions.
Generally, when a signal transmission cable does a high speed signal transmission, the cable lowers the resistance to noise, so that a high speed signal transmission comes to be demanded. However, with this cable, the acceleration of the signal transmission speed may raise the problem of unnecessary radiation (Electromagnetic Interference; EMI). In other words, by this method of signal transmission where the signal wave frequency is high, EMI noise (electromagnetic waves) becomes easily leaked causing noise to enter into the neighboring cables, which is known to cause such adverse effects as malfunctions and transmission loss of signal.
To the contrary, from the idea that if the source of noise generation can be sealed in a metallic film then the noise will not leak, a countermeasure is commonly undertaken whereby the outer circumference of the FFC product, as shown in FIG. 2(a) and FIG. 2(b), is formed with a shield layer 105 wherein multiple conductors 106 are established linearly and any given conductor is connected to the appropriate shield layer 105 which in turn is connected to a ground line (G). However, this shield does not regulate electrical characteristics.
In other words, with this signal transmission cable, the formation of the shield layer as an EMI countermeasure does decrease the problems caused by noise but form the viewpoint of attempting to accelerate the signal transmission speed it is impossible to ignore the effect of transmission loss caused by the inability of impedance matching within the cable. With this cable, reflection may occur in the cable due to the inability of impedance matching, leading to the reflected signal being emitted as noise outside of the cable.
The shield is thought to be one of the causes of this type of reflection. In other words, with this cables it is necessary to use metallic plates or metallic films as a shielding plate in order to prevent noise leakage to the outside. This method is effective as an EMI countermeasure but, from the viewpoint of electrical characteristics, creates inconveniences such as a large increase in electrostatic capacity and a lowering of characteristic impedance due to the existence of a metallic body in proximity to the signal transmission conductor. As a method for lowering this type of electrostatic capacity, physical measures such as decreasing the cross-sectional pile of the conductors, expanding the pitch between the conductors, and expanding the distance between the conductors and metallic bodies are effective but have a large effect on the specifications of the product and cannot be easily changed. Further, due to mobility requirements, the FFC has strict limitations placed on thickness and also when considering the stress placed on the FFC when flexing, a slimmer form is desirable. Of course with an FFC it is conceivable to remove the shield that causes the decrease of impedance, but this would be rash to simply remove the shield due to the effect of the noise.
In the manner described above, with this cable, it especially becomes extremely difficult to make an FFC appropriate for high speed transmission due to the shield equipped as a noise countermeasure, because the shield layer may impair the electrical property of the cable.
Furthermore, with the FFC, there is tested characteristic impedance regulation technology, such as that described in patent document 1.
Patent document 1: Japanese Patent Application Laid-Open No. 2003-31033
Specifically, in this patent document 1, a flexible flat cable equipped with a metallic layer having an attached conductivity adhesive layer wherein a row of multiple conductors arranged in a parallel manner and a foam insulator with an adhesive layer that is laminated after sandwiching this row of conductors from both sides further sandwich a foam insulator having an adhesive agent on both sides is disclosed. In this manner, this flexible flat cable, due to the lamination of the foam insulator having sandwiched the row of conductors at both ends, the dielectric constant of the foam insulator is combined with the dielectric constant of the air allowing the combined dielectric constant to be lower than the dielectric constant of the insulator that is not yet foaming, making it possible to regulate the electrostatic capacity which is the characteristic impedance factor and make the characteristic impedance factor 50 Ohms. Furthermore, with this flexible flat cable the foam insulator is relatively large having a thickness of 150 μm to 250 μm and for the metallic layer having an attached conductivity adhesive layer, a metallic layer laminated with aluminum foil and base film is used.
It is to be noted that many high frequency cables taking into consideration the effect of the shield and electrical characteristics, mostly extremely fine coaxial cables and the like, are being sold, but for a high price and furthermore use specialized connecters which, accompanied by the specialized terminal furnishment necessary for connecting the connecters, require a large amount of wiring production costs and have poor effectiveness making them not generally applicable when compared to the FPC connecters. Furthermore, the high frequency waves are generally classified by MHz bandwidth and GHz bandwidths, but the high frequency cables being sold have specifications that are usable with GHz bandwidths. Because of this regardless of the fact that only the MHz bandwidth is to be used, it is necessary in actuality to use an expensive cable with a GHz bandwidth, imposing a large burden of cost. Also, the technology described above in Patent Document 1, having the objective of regulating the characteristic impedance in general high frequency circuits to an appropriate level of 50 Ohms, is completely inappropriate for machines that require other types of characteristic impedance and differential impedance.
Accordingly, with cables following the FFC, it is anticipated that it will be possible to show high effects from the shield without incurring losses of the electrical characteristics and the desired differential impedance will be able to be realized.